Large multiplier for programmable logic device

ABSTRACT

A plurality of specialized processing blocks in a programmable logic device, including multipliers and circuitry for adding results of those multipliers, can be configured as a larger multiplier by adding to the specialized processing blocks selectable circuitry for shifting multiplier results before adding. In one embodiment, this allows all but the final addition to take place in specialized processing blocks, with the final addition occurring in programmable logic. In another embodiment, additional compression and adding circuitry allows even the final addition to occur in the specialized processing blocks.

BACKGROUND OF THE INVENTION

This invention relates to programmable logic devices (PLDs), and, more particularly, to the use of specialized processing blocks which may be included in such devices to perform large multiplications.

As applications for which PLDs are used increase in complexity, it has become more common to design PLDs to include specialized processing blocks in addition to blocks of generic programmable logic resources. Such specialized processing blocks may include a concentration of circuitry on a PLD that has been partly or fully hardwired to perform one or more specific tasks, such as a logical or a mathematical operation. A specialized processing block may also contain one or more specialized structures, such as an array of configurable memory elements. Examples of structures that are commonly implemented in such specialized processing blocks include: multipliers, arithmetic logic units (ALUs), barrel-shifters, various memory elements (such as FIFO/LIFO/SIPO/RAM/ROM/CAM blocks and register files), AND/NAND/OR/NOR arrays, etc., or combinations thereof.

One particularly useful type of specialized processing block that has been provided on PLDs is a digital signal processing (DSP) block, which may be used to process, e.g., audio signals. Such blocks are frequently also referred to as multiply-accumulate (“MAC”) blocks, because they include structures to perform multiplication operations, and sums and/or accumulations of multiplication operations.

For example, a PLD sold by Altera Corporation, of San Jose, Calif., under the name STRATIX® II includes DSP blocks, each of which includes four 18-by-18 multipliers. Each of those DSP blocks also includes adders and registers, as well as programmable connectors (e.g., multiplexers) that allow the various components to be configured in different ways. In each such block, the multipliers can be configured not only as four individual 18-by-18 multipliers, but also as four smaller multipliers, or as one larger (36-by-36) multiplier. In addition, one 18-by-18 complex multiplication (which decomposes into two 18-by-18 multiplication operations for each of the real and imaginary parts) can be performed.

Although such a DSP block may be configured as a multiplier as large as 36-by-36, a user may want to create a larger multiplier. For example, while a 36-by-36 multiplier will support 25-by-25 single-precision multiplication under the IEEE 754-1985 standard, it is too small for double-precision multiplication. While the multipliers from several DSP blocks can be used together to implement double-precision multiplication, the logic needed to interconnect the multipliers has heretofore been programmed by the user in the general-purpose programmable logic outside the DSP block, making it slow and less efficient, and consuming general-purpose resources that might be put to other uses.

SUMMARY OF THE INVENTION

The present invention relates to specialized processing blocks for PLDs that are provided with logic within the blocks to facilitate the performance of multiplications larger than that which can be performed within any single specialized processing block, reducing or eliminating reliance on general-purpose programmable resources of the PLD.

In one embodiment, additional shifting resources are provided within the specialized processing blocks so that all of the partial products can be computed within the specialized processing blocks, although the final addition of those products occurs outside the specialized processing blocks in general-purpose programmable logic. In another embodiment, additional shifting and adding resources are added to the specialized processing blocks so that substantially the entire multiplication can be carried out without resorting to the general-purpose programmable resources of the PLD.

In accordance with the present invention, there is provided, for use in a programmable logic device having a plurality of specialized processing blocks, each of the specialized processing blocks having at least four n-by-n multipliers arranged in four-multiplier units, a method of performing a 3n-by-3n multiplication operation. The method includes performing a 2n-by-2n multiplication using four of the n-by-n multipliers in a first of the four-multiplier units, performing an n-by-n multiplication using one of the n-by-n multipliers in a second of the four-multiplier units, performing first and second 2n-by-n multiplications in a third of the four-multiplier units, using two of the n-by-n multipliers for each of the 2n-by-n multiplications, shifting a second partial product of each of the 2n-by-n multiplications to align it with a first partial product of each of the 2n-by-n multiplications for addition within the third four-multiplier unit, and adding results of the multiplications from the first, second and third four-multiplier units.

A programmable logic device configured to perform the method, and software to configure the programmable logic device, are also provided.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which:

FIG. 1 is a representation of the decomposition of a 54-bit-by-54-bit multiplication into a sum of partial products;

FIG. 2 is a representation of the alignment of the partial products of FIG. 1 for addition;

FIG. 3 is schematic representation of a portion of a specialized processing block for use in a first preferred embodiment of the present invention;

FIG. 4 is a schematic representation of a the performance of a 54-bit-by-54-bit multiplication in the first preferred embodiment of the present invention;

FIG. 5 is a schematic representation of a group of specialized processing blocks for use in a second preferred embodiment of the present invention;

FIG. 6 is a schematic representation of a 4:2 compressor used in the embodiment of FIG. 5;

FIG. 7 is a simplified block diagram of an illustrative system employing a programmable logic device incorporating the present invention;

FIG. 8 is a cross-sectional view of a magnetic data storage medium encoded with a set of machine-executable instructions for performing the method according to the present invention; and

FIG. 9 is a cross-sectional view of an optically readable data storage medium encoded with a set of machine executable instructions for performing the method according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will now be described with reference to FIGS. 1-6, in the context of a 54-bit-by-54-bit multiplication, which maps well onto the 18-bit multipliers of the DSP block of the aforementioned STRATIX® II PLD, and which can be used to implement double-precision multiplication under the IEEE 754-1985 standard. However, the invention can be used with specialized processing blocks of different sizes.

FIG. 1 shows the decomposition of a 54-bit-by-54-bit multiplication 10 into a sum 11 of partial products 12 that can be implemented using 18-bit-by-18-bit multipliers to yield product 13. In the first multiplicand 101, A contains the 18 most significant bits, and B contains the 36 least significant bits. In the second multiplicand 102, C contains the 18 most significant bits, and C contains the 36 least significant bits. The result (A,B)×(C,D) can be calculated as B×D+((A×D+C×B)<<36)+((A×C)<<72), where “<<n” indicates that the result of the expression to which it relates is shifted to the left by n places.

The intermediate values required for a floating point mantissa multiplication preferably are unsigned when performing a 54-bit multiplication—i.e., they include a 52-bit mantissa preceded by “01.” The intermediate values can be aligned as in FIG. 2, providing as outputs 36-bit output 20 and 3-level 72-bit addition 21.

In the DSP block of the aforementioned STRATIX® II PLD, as well as in an improved DSP block described in copending, commonly-assigned U.S. patent application Ser. Nos. 11/447,329, 11/447,370, 11/447,472, 11/447,474, all filed Jun. 5, 2006, Ser. No. 11/426,403, filed Jun. 26, 2006, and Ser. No. 11/458,361, filed Jul. 18, 2006, each of which is hereby incorporated herein in its respective entirety, four multipliers are arranged in a unit, which may be referred to as a block or a half-block, along with compressors, adders, shifters and multiplexers, to form and add the various partial products.

As applied to the current problem illustrated in FIGS. 1 and 2, that DSP block architecture can support the 36-bit-by-36-bit multiplication (B×D) and the 18-bit-by-18-bit multiplication (A×C), but the multiplexer pattern of that architecture cannot support the connections necessary to add together the two 18-bit-by-36-bit multiplications (A×D and C×B). Each of the 18-bit-by-36-bit multiplications is supported individually, but the results must be routed out of the DSP block, and added in the general-purpose programmable logic of the PLD. This consumes a large amount of general-purpose programmable logic as well as routing and interconnect resources.

In accordance with the present invention, the intermediate multiplexer arrangement of the DSP block is changed, as compared to the aforementioned DSP block, in a manner that allows the sum of two 18-bit-by-36-bit multiplications to be produced in a single four-multiplier block/half-block. As a result, all of the partial products necessary for a 54-bit-by-54-bit multiplication can be performed and at least partially summed together within a single four-multiplier block/half-block.

In a first preferred embodiment illustrated in FIGS. 3 and 4, for a pair of multiplicands A and D, D may be split into most significant and least significant halves, or DH and DL. The product A×D can then be expressed as (A×DH)<<18+A×DL. (A×DH) preferably is provided at 310 by multiplier 31, and is then shifted left 18 bits by shifter 311, selected by multiplexer 312 under control of signal 313. A×DL preferably is provided at 320 by multiplier 32. The product A×D is then preferably provided by adding partial products 310 and 320 at adder 33, which may include a 4:2 compressor, and a 30-bit adder and a 24-bit adder concatenated together (not shown).

A second pair of multiplicands C and B can be treated similarly to provide (C×BH)<<18+C×BL. (C×BH) preferably is provided at 330 by multiplier 33, and is then shifted left 18 bits by shifter 331, selected by multiplexer 332 under control of signal 333. C×BL preferably is provided at 340 by multiplier 34. The product C×B is then preferably provided by adding partial products 330 and 340 at adder 35, which may include a 4:2 compressor, and a 30-bit adder and a 24-bit adder concatenated together (not shown).

The two 54-bit sums of the 18-bit-by-36-bit multiplications A×D and C×B preferably are then added together at adder 36, which may include a 4:2 compressor, and two 44-bit adders concatenated together (not shown). Although a 18-bit shifter 37 is provided for selectively left-shifting the output of adder 33 as selected by multiplexer 370 under control of signal 371, for the purpose of this 54-bit addition, sum 33 (A×D) is not shifted.

Specifically, the three shifters 311, 331, 37, under control of signals 312, 332, 371, allows specialized processing block 30 to be used for multiple functions. For example, for a sum of four 18-bit-by-18-bit multiplications, each of signals 312, 332, 371 preferably is set to select its respective unshifted result. For a single 36-bit-by-36-bit multiplication, each of signals 312, 332, 371 preferably is set to select its respective shifted result. And as already stated, for performing the two 18-bit-by-36-bit partial products of a 54-bit-by-54-bit multiplication, each of signals 312, 332 preferably is set to select its respective shifted result, while signal 371 preferably is set to select its unshifted result.

As seen in FIG. 4, the 54-bit-by-54-bit multiplication is performed by using specialized processing block/half-block 40 to perform the 36-bit-by-36-bit partial product B×D, using specialized processing block/half-block 30 to perform and sum the two 18-bit-by-36-bit partial products A×D and C×B, and using specialized processing block/half-block 41 to perform the single 18-bit-by-18-bit multiplication A×C. Note that only one of the four multipliers 410-413 in block/half-block 41 is used, although as explained in above-incorporated application Ser. No. 11/447,472, if block/half-block 41 is the one described in that application, using only one multiplier 410 requires sacrificing a second multiplier 411. However, in that embodiment, at least multipliers 412, 413 remain available for other purposes, and in other embodiments even multiplier 411 may be available.

In accordance with the embodiment of the present invention depicted in FIGS. 3 and 4, the three partial products or sums of partial products 405, 305 and 415 are added by adder 42, which preferably is created outside the specialized processing blocks 40, 30, 41 in programmable logic of the PLD of which specialized processing blocks 40, 30, 41 are a part.

In the embodiment of FIGS. 3 and 4, it is still necessary to use general-purpose programmable logic, routing and interconnect resources for the final addition 42. In a second preferred embodiment 50 shown in FIG. 5, a 54-bit-by-54-bit multiplication can be performed substantially entirely in specialized processing blocks on a PLD, substantially without resort to the general-purpose programmable logic of that PLD. In embodiment 50, preferably two four-multiplier units 51, 52 and a portion of third four-multiplier unit 53 are used. Preferably, each of these four-multiplier units 51-53 is based on half-blocks of the specialized processing block described in above-incorporated application Ser. No. 11/447,472, modified as described herein. Thus, a full one such block and a portion of a second such block preferably are used.

In embodiment 50, each half-block 51, 52 (and half-block 53, but not all components are shown because only one multiplier 530 is used from that half-block 52) preferably has four 18-bit-by-bit multipliers 510-513, 520-523, preferably arranged in pairs 510-511, 512-513, 520-521 and 522-523, with the output of the members of each pair preferably being added together by respective 54-bit adders 541-544 after the output of one member of pair has been shifted left 18 bits by respective shifter 55. One or more of shifters 55 may be programmably bypassable (not shown) as in the embodiment of FIGS. 3 and 4, above, but in this embodiment, for performing a 54-bit-by-54-bit multiplication, shifters 55 preferably are not bypassed (even if they are bypassable).

In the specialized processing block described in above-incorporated application Ser. No. 11/447,472, the output of adder 541, and the output of adder 542 after being shifted left 18 bits by shifter 545, would be added by 3:2 compressor 560 and chained carry/propagate adders 570, 571. Similarly, the outputs of adders 543 and 544 would be added by 3:2 compressor 561 and chained carry/propagate adders 572, 573. In accordance with the present invention, a 4:2 compressor 562 as well as two 36-bit right-shifters 546, 547 are added. A number of AND gates 580-583 are added as selectors as described below, although multiplexers also could be used for that purpose, and AND gate 584 is added to chain together adders 570, 571 with adders 572, 573. In addition, 18-bit right-shifter 548 and AND gate 585 are added, bridging half-blocks 52, 53 which are in different specialized processing blocks. Note that a further 18-bit right-shifter (not shown) like shifter 548 and a further AND gate (not shown) like AND gate 585, could connect half-block 51 to another half-block to the right (not shown) in a similar manner.

When not being used in the 54-bit-by-54-bit multiplication mode, each specialized processing block operates like that shown in above-incorporated application Ser. No. 11/447,472. As such, the second input (not shown) of each of AND gates 580, 582, 584 and 585 is a “0” so that shifters 546-548 are not in use and the carry/propagate adder chains of the two half-blocks remain separate. Similarly, the second input (not shown) of each of AND gates 581, 583 is a “1” so that each partial product feeds directly into its respective 3:2 or 4:2 compressor. Note that in this case, with a “0” on the second input of AND gate 580, 4:2 compressor 562 will act like a 3:2 compressor 560, 561.

When the specialized processing blocks are being used in the 54-bit-by-54-bit multiplication mode, the second input (not shown) of each of AND gates 580, 582, 584 and 585 is a “1” so that shifters 546-548 are in use and the carry/propagate adder chains of the two half-blocks are connected. Because this is a 72-bit addition, the carry-out from 44-bit adder 571 to 44-bit adder 572 (via AND gate 584) preferably is taken not from the end of adder 571, but preferably from the 29th bit of adder 571, which, including adder 570, is the 73rd bit position, representing the carry-out from a 72-bit addition. Although it relies on more than one specialized processing block, this arrangement adds together all of the partial products substantially without resorting to general-purpose programmable logic of the PLD.

FIG. 6 shows schematically how 4:2 compressor 562 may be configured from two 3:2 compressors 560 (or 561).

Thus it is seen that a large multiplication that requires more than one specialized processing block of a PLD can be performed using fewer or no general-purpose programmable resources of the PLD.

A PLD 280 incorporating such circuitry according to the present invention may be used in many kinds of electronic devices. One possible use is in a data processing system 900 shown in FIG. 7. Data processing system 900 may include one or more of the following components: a processor 281; memory 282; I/O circuitry 283; and peripheral devices 284. These components are coupled together by a system bus 285 and are populated on a circuit board 286 which is contained in an end-user system 287.

System 900 can be used in a wide variety of applications, such as computer networking, data networking, instrumentation, video processing, digital signal processing, or any other application where the advantage of using programmable or reprogrammable logic is desirable. PLD 280 can be used to perform a variety of different logic functions. For example, PLD 280 can be configured as a processor or controller that works in cooperation with processor 281. PLD 280 may also be used as an arbiter for arbitrating access to a shared resources in system 900. In yet another example, PLD 280 can be configured as an interface between processor 281 and one of the other components in system 900. It should be noted that system 900 is only exemplary, and that the true scope and spirit of the invention should be indicated by the following claims.

Various technologies can be used to implement PLDs 280 as described above and incorporating this invention.

Instructions for carrying out the method according to this invention may be encoded on a machine-readable medium, to be executed by a suitable computer or similar device to implement the method of the invention for programming PLDs. For example, a personal computer may be equipped with an interface to which a PLD can be connected, and the personal computer can be used by a user to program the PLD using a suitable software tool, such as the QUARTUS® II software available from Altera Corporation, of San Jose, Calif.

FIG. 8 presents a cross section of a magnetic data storage medium 600 which can be encoded with a machine executable program that can be carried out by systems such as the aforementioned personal computer, or other computer or similar device. Medium 600 can be a floppy diskette or hard disk, or magnetic tape, having a suitable substrate 601, which may be conventional, and a suitable coating 602, which may be conventional, on one or both sides, containing magnetic domains (not visible) whose polarity or orientation can be altered magnetically. Except in the case where it is magnetic tape, medium 600 may also have an opening (not shown) for receiving the spindle of a disk drive or other data storage device.

The magnetic domains of coating 602 of medium 600 are polarized or oriented so as to encode, in manner which may be conventional, a machine-executable program, for execution by a programming system such as a personal computer or other computer or similar system, having a socket or peripheral attachment into which the PLD to be programmed may be inserted, to configure appropriate portions of the PLD, including its specialized processing blocks, if any, in accordance with the invention.

FIG. 9 shows a cross section of an optically-readable data storage medium 700 which also can be encoded with such a machine-executable program, which can be carried out by systems such as the aforementioned personal computer, or other computer or similar device. Medium 700 can be a conventional compact disk read only memory (CD-ROM) or digital video disk read only memory (DVD-ROM) or a rewriteable medium such as a CD-R, CD-RW, DVD-R, DVD-RW, DVD+R, DVD+RW, or DVD-RAM or a magneto-optical disk which is optically readable and magneto-optically rewriteable. Medium 700 preferably has a suitable substrate 701, which may be conventional, and a suitable coating 702, which may be conventional, usually on one or both sides of substrate 701.

In the case of a CD-based or DVD-based medium, as is well known, coating 702 is reflective and is impressed with a plurality of pits 703, arranged on one or more layers, to encode the machine-executable program. The arrangement of pits is read by reflecting laser light off the surface of coating 702. A protective coating 704, which preferably is substantially transparent, is provided on top of coating 702.

In the case of magneto-optical disk, as is well known, coating 702 has no pits 703, but has a plurality of magnetic domains whose polarity or orientation can be changed magnetically when heated above a certain temperature, as by a laser (not shown). The orientation of the domains can be read by measuring the polarization of laser light reflected from coating 702. The arrangement of the domains encodes the program as described above.

It will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications can be made by those skilled in the art without departing from the scope and spirit of the invention. For example, the various elements of this invention can be provided on a PLD in any desired number and/or arrangement. One skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments, which are presented for purposes of illustration and not of limitation, and the present invention is limited only by the claims that follow. 

1. For use in a programmable logic device having a plurality of specialized processing blocks, each of said specialized processing blocks having at least four n-by-n multipliers arranged in four-multiplier units, a method of performing a 3n-by-3n multiplication operation, said method comprising: performing a 2n-by-2n multiplication using four of said n-by-n multipliers in a first of said four-multiplier units; performing an n-by-n multiplication using one of said n-by-n multipliers in a second of said four-multiplier units; performing first and second 2n-by-n multiplications in a third of said four-multiplier units, using two of said n-by-n multipliers for each of said 2n-by-n multiplications; shifting a second partial product of each of said 2n-by-n multiplications to align it with a first partial product of each of said 2n-by-n multiplications for addition within said third four-multiplier unit; and adding results of said multiplications from said first, second and third four-multiplier units.
 2. The method of claim 1 wherein said adding comprises adding said results in general-purpose programmable logic of said programmable logic device.
 3. The method of claim 2 wherein: said performing first and second 2n-by-n multiplications comprises, for each respective one of said first and second 2n-by-n multiplications: performing a respective most significant bit multiplication using one said multiplier in said third four-multiplier unit to form a respective most significant bit partial product, and performing a respective least significant bit multiplication using another said multiplier in said third four-multiplier unit to form a respective least significant bit partial product; said shifting comprises shifting each respective most significant bit partial product to the left without shifting either respective least significant bit partial product; and said addition within said third four-multiplier unit excludes further shifting of partial products.
 4. The method of claim 3 further comprising selecting control signals to perform said shifting and said addition without further shifting.
 5. The method of claim 1 wherein each said specialized processing block comprises two said four-multiplier units.
 6. The method of claim 5 wherein said adding comprises performing said adding substantially in one said specialized processing block.
 7. The method of claim 6 wherein: both said 2n-by-2n multiplication and said first and second 2n-by-n multiplications are performed in said one specialized processing block comprising said first and third four-multiplier units; said performing said 2n-by-2n multiplication and first and second 2n-by-n multiplications comprises, for each respective one of said 2n-by-2n multiplication and first and second 2n-by-n multiplications: performing a most significant bit multiplication for said 2n-by-2n multiplication using one said multiplier in said first four-multiplier unit to form a most significant bit partial product for said 2n-by-2n multiplication, and performing a respective most significant bit multiplication for each respective one of said 2n-by-n multiplications using a respective one of said multipliers in said third four-multiplier unit to form a respective most significant bit partial product for each respective one of said 2n-by-n multiplications, and performing a least significant bit multiplication for said 2n-by-2n multiplication using another said multiplier in said first four-multiplier unit to form a least significant bit partial product for said 2n-by-2n multiplication, and performing a respective least significant bit multiplication for each respective one of said 2n-by-n multiplications using respective others of said multipliers in said third four-multiplier unit to form a respective least significant bit partial product for each respective one of said 2n-by-n multiplications; and said shifting comprises shifting each respective one of said most significant bit partial products of said 2n-by-n multiplications to the left without shifting either respective one of said least significant bit partial products of said 2n-by-n multiplications; said method further comprising: shifting said most significant bit partial product of said 2n-by-2n multiplication to the left without shifting said least significant bit partial product of said 2n-by-2n multiplication; summing said most significant and least significant bit partial products of said 2n-by-2n multiplication to produce a most significant bit sum and a least significant bit sum of said 2n-by-2n partial products of said 2n-by-2n multiplication; summing each pair of respective most significant and least significant bit partial products of said 2n-by-n multiplications to produce a respective most significant bit sum and least significant bit sum for each of said 2n-by-n multiplications; left-shifting said most significant bit sum of said 2n-by-2n partial products of said 2n-by-2n multiplication; right-shifting each of said most significant bit sum of said 2n-by-n partial products and said least significant bit sum of said 2n-by-n partial products; right-shifting output of said n-by-n multiplication and inputting said right-shifted output to said one of said specialized processor blocks comprising said first and third four-multiplier units; and adding said left-shifted most significant bit sum of said 2n-by-2n partial products, said right-shifted most and least significant bit sums of said 2n-by-n partial products, said right-shifted output of said n-by-n multiplication, and said least significant bit sum of said 2n-by-2n partial products.
 8. The method of claim 7 wherein adding said left-shifted most significant bit sum of said 2n-by-2n partial products, said right-shifted most and least significant bit sums of said 2n-by-n partial products, said right-shifted output of said n-by-n multiplication, and said least significant bit sum of said 2n-by-2n partial products comprises: compressing said least significant bit sum of said 2n-by-2n partial products; compressing said left-shifted most significant bit sum of said 2n-by-2n partial products with a first one of said right-shifted most and least significant bit sums of said 2n-by-n partial products; compressing said right-shifted output of said n-by-n multiplication with a second one of said right-shifted most and least significant bit sums of said 2n-by-n partial products; and adding results of said compressings.
 9. A programmable logic device having a plurality of specialized processing blocks, each of said specialized processing blocks having at least four n-by-n multipliers arranged in four-multiplier units, said programmable logic device being configured to perform a 3n-by-3n multiplication operation and comprising: four of said n-by-n multipliers in a first of said four-multiplier units configured to perform a 2n-by-2n multiplication; one of said n-by-n multipliers in a second of said four-multiplier units configured to perform an n-by-n multiplication; a third of said four-multiplier units configured to perform first and second 2n-by-n multiplications, using two of said n-by-n multipliers for each of said 2n-by-n multiplications; a shifter configured to shift a second partial product of each of said 2n-by-n multiplications to align it with a first partial product of each of said 2n-by-n multiplications for addition within said third four-multiplier unit; and circuitry configured to add results of said multiplications from said first, second and third four-multiplier units.
 10. The configured programmable logic device of claim 9 wherein said adding comprises adding said results in general-purpose programmable logic of said programmable logic device.
 11. The configured programmable logic device of claim 10 wherein: said programmable logic device is configured to perform said first and second 2n-by-n multiplications by, for each respective one of said first and second 2n-by-n multiplications: performing a respective most significant bit multiplication using one said multiplier in said third four-multiplier unit to form a respective most significant bit partial product, and performing a respective least significant bit multiplication using another said multiplier in said third four-multiplier unit to form a respective least significant bit partial product; said programmable logic device is configured to shift each respective most significant bit partial product to the left without shifting either respective least significant bit partial product; and said circuitry configured to add excludes further shifting of partial products.
 12. The configured programmable logic device of claim 11 further comprising selectors responsive to selection control signals to perform said shifting and said addition without further shifting.
 13. The configured programmable logic device of claim 9 wherein each said specialized processing block comprises two said four-multiplier units.
 14. The configured programmable logic device of claim 13 wherein said circuitry configured to add is located substantially within one said specialized processing block.
 15. The configured programmable logic device of claim 14 wherein: said one specialized processing block comprises both said first four-multiplier unit configured to perform said 2n-by-2n multiplication and said third four-multiplier unit configured to perform said first and second 2n-by-n multiplications; in each of said first and third four-multiplier units, said performing said 2n-by-2n multiplication and first and second 2n-by-n multiplications comprises, for each respective one of said 2n-by-2n multiplication and first and second 2n-by-n multiplications: performing a most significant bit multiplication for said 2n-by-2n multiplication using one said multiplier in said first four-multiplier unit to form a most significant bit partial product for said 2n-by-2n multiplication, and performing a respective most significant bit multiplication for each respective one of said 2n-by-n multiplications using a respective one of said multipliers in said third four-multiplier unit to form a respective most significant bit partial product for each respective one of said 2n-by-n multiplications, and performing a least significant bit multiplication for said 2n-by-2n multiplication using another said multiplier in said first four-multiplier unit to form a least significant bit partial product for said 2n-by-2n multiplication, and performing a respective least significant bit multiplication for each respective one of said 2n-by-n multiplications using respective others of said multipliers in said third four-multiplier unit to form a respective least significant bit partial product for each respective one of said 2n-by-n multiplications; and said shifter is configured to shift each respective one of said most significant bit partial products of said 2n-by-n multiplications to the left without shifting either respective one of said least significant bit partial products of said 2n-by-n multiplications; said configured programmable logic device further comprising: a shifter to shift said most significant bit partial product of said 2n-by-2n multiplication to the left without shifting said least significant bit partial product of said 2n-by-2n multiplication; circuitry configured to sum said most significant and least significant bit partial products of said 2n-by-2n multiplication to produce a most significant bit sum and a least significant bit sum of said 2n-by-2n partial products of said 2n-by-2n multiplication; circuitry configured to sum each pair of respective most significant and least significant bit partial products of said 2n-by-n multiplications to produce a respective most significant bit sum and least significant bit sum for each of said 2n-by-n multiplications; first left-shifting circuitry to left-shift said most significant bit sum of said 2n-by-2n partial products of said 2n-by-2n multiplication; right-shifting circuitry to right-shift said most significant bit sum of said 2n-by-n partial products and said least significant bit sum of said 2n-by-n partial products; second right-shifting circuitry to right-shift output of said n-by-n multiplication and input said right-shifted output of said n-by-n multiplication to said one of said specialized processor blocks comprising said first and third four-multiplier units; and circuitry configured to add said left-shifted most significant bit sum of said 2n-by-2n partial products, said right-shifted most and least significant bit sums of said 2n-by-n partial products, said right-shifted output of said n-by-n multiplication, and said least significant bit sum of said 2n-by-2n partial products.
 16. The configured programmable logic device of claim 15 wherein circuitry configured to add said left-shifted most significant bit sum of said 2n-by-2n partial products, said right-shifted most and least significant bit sums of said 2n-by-n partial products, said left-shifted output of said n-by-n multiplication, and said least significant bit sum of said 2n-by-2n partial products comprises: first 3:2 compression circuitry to compress said least significant bit sum of said 2n-by-2n partial products; 4:2 compression circuitry to compress said left-shifted most significant bit sum of said 2n-by-2n partial products with a first one of said right-shifted most and least significant bit sums of said 2n-by-n partial products; second 3:2 compression circuitry to compress said right-shifted output of said n-by-n multiplication with a second one of said right-shifted most and least significant bit sums of said 2n-by-n partial products; and adding circuitry to add outputs of said first and second 3:2 compression circuitry and said 4:2 compression circuitry.
 17. A data storage medium encoded with machine-executable instructions for performing a method of programmably configuring a programmable logic device to perform a 3n-by-3n multiplication operation, wherein said programmable logic device has a plurality of specialized processing blocks, each of said specialized processing blocks having at least four n-by-n multipliers arranged in four-multiplier units, said instructions comprising: instructions for configuring four of said n-by-n multipliers in a first of said four-multiplier units to perform a 2n-by-2n multiplication; instructions for configuring one of said n-by-n multipliers in a second of said four-multiplier units to perform an n-by-n multiplication; instructions for configuring a third of said four-multiplier units to perform first and second 2n-by-n multiplications, using two of said n-by-n multipliers for each of said 2n-by-n multiplications; instructions for configuring a shifter to shift a second partial product of each of said 2n-by-n multiplications to align it with a first partial product of each of said 2n-by-n multiplications for addition within said third four-multiplier unit; and instructions for configuring circuitry to add results of said multiplications from said first, second and third four-multiplier units.
 18. The data storage medium of claim 17 wherein said instructions for configuring circuitry to add comprise instructions for configuring general-purpose programmable logic of said programmable logic device to add said results.
 19. The data storage medium of claim 18 comprising: instructions to configure said programmable logic device to perform said first and second 2n-by-n multiplications including, for each respective one of said first and second 2n-by-n multiplications: instructions to configure said programmable logic device to perform a respective most significant bit multiplication using one said multiplier in said third four-multiplier unit to form a respective most significant bit partial product, and instructions to configure said programmable logic device to perform a respective least significant bit multiplication using another said multiplier in said third four-multiplier unit to form a respective least significant bit partial product; and instructions to configure said programmable logic device to perform to shift each respective most significant bit partial product to the left without shifting either respective least significant bit partial product; wherein: said instructions to configure said circuitry to add excludes further shifting of partial products.
 20. The data storage medium of claim 19 wherein said instructions further comprise instructions to configure selectors responsive to selection control signals to perform said shifting and said addition without further shifting.
 21. The data storage medium of claim 17 wherein said instructions are for configuring a programmable logic device wherein each said specialized processing block comprises two said four-multiplier units.
 22. The data storage medium of claim 21 wherein said instructions configure said circuitry to add substantially within one said specialized processing block.
 23. The data storage medium of claim 22 wherein: said instructions configure both said first and third four-multiplier units configured to perform said 2n-by-2n multiplication and said first and second 2n-by-n multiplications; said instructions configure each of said first and third four-multiplier units to perform said 2n-by-2n multiplication and first and second 2n-by-n multiplications comprises, wherein, for each respective one of said 2n-by-2n multiplication and first and second 2n-by-n multiplications: said instructions configure one said multiplier in said four-multiplier unit to perform a most significant bit multiplication for said 2n-by-2n multiplication to form a respective most significant bit partial product for said 2n-by-2n multiplication, and configure a respective one of said multipliers in said third four-multiplier unit to perform a respective most significant bit multiplication for each respective one of said 2n-by-n multiplications to form a respective most significant bit partial product for each respective one of said 2n-by-n multiplications, and said instructions configure another said multiplier in said four-multiplier unit to perform a least significant bit multiplication for said 2n-by-2n multiplication to form a respective least significant bit partial product for said 2n-by-2n multiplication, and configure respective others of said multipliers in said third four-multiplier unit to perform a respective least significant bit multiplication for each respective one of said 2n-by-n multiplications to form a respective least significant bit partial product for each respective one of said 2n-by-n multiplications; and said instructions configure said shifter to shift each respective one of said most significant bit partial products of said 2n-by-n multiplications to the left without shifting either respective one of said least significant bit partial products of said 2n-by-n multiplications; said instructions configure a shifter to shift said most significant bit partial product of said 2n-by-2n multiplication to the left without shifting said least significant bit partial product of said 2n-by-2n multiplication; said instructions configure circuitry to sum said most significant and least significant bit partial products of said 2n-by-2n multiplication to produce a most significant bit sum and a least significant bit sum of said 2n-by-2n partial products of said 2n-by-2n multiplication; said instructions configure circuitry to sum each pair of respective most significant and least significant bit partial products of said 2n-by-n multiplications to produce a respective most significant bit sum and least significant bit sum of said 2n-by-n multiplications; said instructions configure first left-shifting circuitry to left-shift said most significant bit sum of said 2n-by-2n partial products of said 2n-by-2n multiplication; said instructions configure right-shifting circuitry to right-shift said most significant bit sum of said 2n-by-n partial products and said least significant bit sum of said 2n-by-n partial products; said instructions configure second right-shifting circuitry to right-shift output of said n-by-n multiplication and input said right-shifted output of said n-by-n multiplication to said one of said specialized processor blocks comprising said first and third four-multiplier units; and said instructions configure circuitry to add said left-shifted most significant bit sum of said 2n-by-2n partial products, said right-shifted most and said least significant bit sums of said 2n-by-n partial products, said right-shifted output of said n-by-n multiplication, and said least significant bit sum of said 2n-by-2n partial products.
 24. The data storage medium of claim 23 wherein said instructions to configure circuitry configured to add said left-shifted most significant bit sum of said 2n-by-2n partial products, said right-shifted most and said least significant bit sums of said 2n-by-n partial products, said right-shifted output of said n-by-n multiplication, and said least significant bit sum of said 2n-by-2n partial products comprises: instructions to configure first 3:2 compression circuitry to compress said least significant bit sum of said 2n-by-2n partial products; instructions to configure 4:2 compression circuitry to compress said left-shifted most significant bit sum of said 2n-by-2n partial products with a first one of said right-shifted most and said least significant bit sums of said 2n-by-n partial products; instructions to configure second 3:2compression circuitry to compress said right-shifted output of said n-by-n multiplication with a second one of said right-shifted most and said least significant bit sums of said 2n-by-n partial products; and instructions to configure adding circuitry to add outputs of said first and second 3:2 compression circuitry and said 4:2 compression circuitry. 